1. Field of Invention
The present invention relates generally to heat transfer tubes, their method of formation and use. More particularly, the present invention relates to an improved boiling tube, a method of manufacture and use of that tube in an improved refrigerant evaporator or chiller.
2. General Background
A component device of industrial air conditioning and refrigeration systems is a refrigerant evaporator or chiller. In simple terms, chillers remove heat from a cooling medium that enters the unit, and deliver refreshed cooling medium to the air conditioning or refrigeration system to effect cooling of a structure, device or given area. Refrigerant evaporators on chillers use a liquid refrigerant or other working fluid to accomplish this task. Refrigerant evaporators on chillers lower the temperature of a cooling medium, such as water (or some other fluid), below that which could be obtained from ambient conditions for use by the air conditioning or refrigeration system.
One type of a chiller is a flooded chiller. In flooded chiller applications, a plurality of heat transfer tubes are fully submerged in a pool of a two-phase boiling refrigerant. The refrigerant is often a chlorinated-fluorinated hydrocarbon (i.e., “Freon”) having a specified boiling temperature. A cooling medium, often water, is processed by the chiller. The cooling medium enters the evaporator and is delivered to the plurality of tubes, which are submerged in a boiling liquid refrigerant. As a result, such tubes are commonly known as “boiling tubes.” The cooling medium passing through the plurality of tubes is chilled as it gives up its heat to the boiling refrigerant. The vapor from the boiling refrigerant is delivered to a compressor which compresses the vapor to a higher pressure and temperature. The high pressure and temperature vapor is then routed to a condenser where it is condensed for eventual return through an expansion device to the evaporator to lower the pressure and temperature. Those of ordinary skill in the art will appreciate that the foregoing occurs in keeping with the well-known refrigeration cycle.
It is known that heat transfer performance of a boiling tube submerged in a refrigerant can be enhanced by forming fins on the outside surface of the tube. It is also known to enhance the heat transfer ability of a boiling tube by modifying the inner tube surface that contacts the cooling medium. One example of such a modification to the inner tube surface is shown in U.S. Pat. No. 3,847,212, to Wither, Jr., et al., which teaches forming ridges on a tube's inner surface.
It is further known that the fins can be modified to further enhance heat transferability. For example, some boiling tubes have come to be referred to as nucleate boiling tubes. The outer surface of nucleate boiling tubes are formed to produce multiple cavities or pores (often referred to as boiling or nucleation sites) that provide openings which permit small refrigerant vapor bubbles to be formed therein. The vapor bubbles tend to form at the base or root of the nucleation site and grow in size until they break away from the outer tube surface. Upon breaking away, additional liquid refrigerant takes the vacated space and the process is repeated to form other vapor bubbles. In this manner, the liquid refrigerant is boiled off or vaporized at a plurality of nucleate boiling sites provided on the outer surface of the metallic tubes.
U.S. Pat. No. 4,660,630 to Cunningham et al. shows nucleate boiling cavities or pores formed by notching or grooving fins on the outer surface of the tube. The notches are formed in a direction essentially perpendicular to the plane of the fins. The inner tube surface includes helical ridges. This patent also discloses a cross-grooving operation that deforms the fin tips such that nucleate boiling cavities (or channels) are formed having a greater width than the surface openings. This construction permits the vapor bubbles to travel outwardly through the cavity, to and through the narrower surface openings, which further enhances heat transferability. Various tubes produced in accordance with the Cunningham et al. patent have been marketed by Wolverine Tube, Inc. under the trademark TURBO-B®. In another nucleate boiling tube, marketed under the trademark TURBO-BII®, the notches are formed at an acute angle to the plane of the fins.
In some heat transfer tubes, the fins are rolled over and/or flattened after they are formed so as to produce narrow gaps which overlie the larger cavities or channels defined by the roots of the fins and the sides of adjacent pairs of fins. Examples include the tubes of the following United States patents: Cunningham et al U.S. Pat. No. 4,660,630; Zohler U.S. Pat. No. 4,765,058; Zohler U.S. Pat. No. 5,054,548; Nishizawa et al U.S. Pat. No. 5,186,252; Chiang et al U.S. Pat. No. 5,333,682.
Controlling the density and size of nuclear boiling pores has been recognized in the prior art. Moreover, the interrelationship between pore size and refrigerant type has also been recognized in the prior art. For example, U.S. Pat. No. 5,146,979 to Bohler purports to increase performance using higher pressure refrigerants by employing tubes having nucleate boiling pores ranging in size from 0.000220 square inches to 0.000440 square inches (the total area of the pods being from 14% to 28% of the total outer surface area). In another example, U.S. Pat. No. 5,697,430 to Thors et al. also discloses a heat transfer tube having a plurality of radially outwardly extending helical fins. The tube inner surface has a plurality of helical ridges. The fins of the outer surface are notched to provide nucleate boiling sites having pores. The fins and notches are spaced to provide pores having an average area less than 0.00009 square inches and a pore density of at least 2000 per square inch of the tube's outer surface. The helical ridges on the inner surface have a predetermined ridge height and pitch, and are positioned at a predetermined helix angle. Tubes made in accordance with the inventions of that patent have been offered and sold under the trademark TURBO BIII®.
The industry continues to explore new and improved designs by which to enhance heat transfer and chiller performance. For example, U.S. Pat. No. 5,333,682 discloses a heat transfer tube having an external surface configured to provide both an increased area of the tube's external surface and to provide re-entrant cavities as nucleation sites to promote nucleate boiling. Similarly, U.S. Pat. No. 6,167,950 discloses a heat transfer tube for use in a condenser with notched and finned surfaces configured to promote drainage of refrigerant from the fin. As shown by such developments in the art, it remains a goal to increase the heat transfer performance of nucleate boiling tubes while maintaining manufacturing cost and refrigeration system operation costs at minimum levels. These goals include the design of more efficient tubes and chillers, and methods of manufacturing such tubes. Consistent with such goals, the present invention is directed to improving the performance of heat exchange tubes generally and, in particular, the performance of heat exchange tubes used in flooded chillers or falling film applications.